Insulation question - fiberglass only or fg plus xps

zendogMay 12, 2013

Hi,

I'm looking for some feedback on the insulation we are considering in a 2-story addition for a 1938 colonial in the northern Virginia area (outside DC). The walls of the current home are brick and block with plaster, so maybe an R4 at the most. And we still have the original windows. The exterior of the existing house is 26X30 and we are going to add about 850 square feet off the back (425 on each floor) with the addition running the full length of the house (30 feet).

Exterior walls will be 2x6 and we are looking at either r-19 fiberglass or using a combination of 2 inch XPS foam plus r-13 fiberglass. The roof on the addition will be r-38 fiberglass or using 2 inch xps plus fiberglass. Although the foam increase the r-value and helps tighten up the envelope, it does add a good bit of cost and I'm not sure it is totally worth it. Now if we were just building an entirely new home, that might be different, but super insulation an addition on a basically un-insulated home seems questionable. But it is what one contractor is suggesting - saying it will make it much more air tight and is cheaper than going with the spray in foam option (icynene for instance).

As part of the project we do plan to replace the windows in the existing home and will be insulating/finishing the basement so everything should be getting a little more energy efficient.

What are your thoughts? Are there any health concerns as well? What about issues in case of a fire?

Exterior walls will be 2x6 and we are looking at either r-19 fiberglass or using a combination of 2 inch XPS foam plus r-13 fiberglass. The roof on the addition will be r-38 fiberglass or using 2 inch xps plus fiberglass.

I would sheath with 1" XPS--2" is overkill in your mild climate-- followed by fg batts or blown cellulose. Using XPS as sheathing while bracing or bucking for structural support saves the added cost of wood sheathing.

Blown cellulose in the attic will air seal better than fg.

Limit the spray foam to the rims on all floors.

Local costs will help determine which combination is most efficient for your home.

I'll post this pic again of 1" tongue and groove (shiplap) XPS as sheathing; high density fg batts between the 2x6 studs.

Thanks for your detailed response. Sorry I've been slow to follow up but the day job gets in the way...

My contractor just began digging for the crawl space today (I'll post a question about that separately). Since we already have approved plans, I wasn't sure if using the XPS in place of sheathing will fly with the county, but the contractor is also concerned with applying the hardy board siding over the foam. He says because of the weight of the siding and the softness of the foam it creates problems. He is suggesting we put the foam between studs and also suggests we consider spray in foam. Thoughts on any of this?

I think the cellulose for the cathedral ceiling in particular is a great option. We'll look at the non-vented tight packed option.

The point of applying foam board over the studs, rather than between them, is to eliminate thermal bridging.

See James Hardie Technical Bulletin 19 for details on how to install Hardie Board over continuous foam insulation.

cellulose for the cathedral ceiling

A cathedral ceiling wasn't mentioned in the original post. Dr. Lstiburek of Building Science Corp. recommends against dense pack cellulose in a cathedral or flat roof. (See further discussion and contra opinion from a cellulose insulation representativehere.)

Thanks again Worthy! More great information. I'll bring this up with my contractor tomorrow.

By the way, I noticed Hardy talked about 1/2 inch and maybe that would be adequate. Any idea if the foil faced Polyisocyanurate stuff compares to XPS in this type of application. I don't see 1/2 XPS in sheets (maybe because it is so brittle when thin).

The foil-faced polyiso is more suited for a hot-humid climate due to its high level of impermeance vs. XPS. In a mixed-humid climate such as yours, you have to consider the drying strategy. (See link.)

I agree, I wouldn't use less than an inch of XPS due to the flimsiness of 1/2" boards.